The present invention relates to the use of power control techniques in cellular radio telephone communication systems, and more particularly, to methods and systems related to power control for base stations.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
FIG. 1 illustrates an example of a conventional cellular radio communication system 100. The radio communication system 100 includes a plurality of radio base stations 170a-n connected to a plurality of corresponding antennas 130a-n. The radio base stations 170a-n in conjunction with the antennas 130a-n communicate with a plurality of mobile stations (e.g. stations 120a, 120b and 120m) within a plurality of cells 110a-n. Communication from a base station to a mobile station is referred to as the downlink, whereas communication from a mobile station to the base station is referred to as the uplink.
The base stations are connected to a mobile switching center (MSC) 150. Among other tasks, the MSC coordinates the activities of the base stations, such as during the handoff of a mobile station from one cell to another. The MSC, in turn, can be connected to a public switched telephone network 160, which services various communication devices 180a, 180b and 180c. 
Power control techniques have been implemented in radiocommunication systems to ensure reliable reception of a signal at each mobile station, i.e., to provide a signal to interference ratio (SIR) above a prescribed threshold for each mobile station. To improve the reception of a mobile station whose SIR drops below this threshold, the energy of the signal may be increased to appropriate levels. However, increasing the energy associated with one mobile station increases the interference associated with other nearby mobile stations which may be located in the same cell or in nearby cells. As such, the radio communication system must strike a balance between the requirements of all mobile stations, located in the same cell and in nearby cells, sharing the same common channel and adjacent channels. A steady state condition is reached when the SIR requirements for all mobile stations within a given radio communication system are satisfied. Generally speaking, the balanced steady state condition may be achieved by using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high power levels increases interference experienced at each mobile receiver, and limits the number of signals which may be successfully communicated on the common channel and on adjacent channels (e.g. reduces system capacity).
This technique for controlling transmit power in radiocommunication systems is commonly referred to as a fast power control loop. The initial SIR target is established based upon a desired quality of service (QoS) for a particular connection or service type. For non-orthogonal channels, the actual SIR values experienced by a particular mobile station or base station can be expressed as:                     SIR        =                              Mean            ⁢                          xe2x80x83                        ⁢            power            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            received            ⁢                          xe2x80x83                        ⁢            signal                                Sum            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            mean            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            powers            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            all            ⁢                          xe2x80x83                        ⁢            interfering            ⁢                          xe2x80x83                        ⁢            signals                                              (        1        )            
The SIR is measured by the receiving party and is used for determining which power control command is sent to the transmitting party.
A slow power control loop can then be used to adjust the SIR target value on an ongoing basis. For example, the mobile station can measure the quality of the signals received from the base station using, for example, known bit error rate (BER) or frame error rate (FER) techniques. Based upon the received signal quality, which may fluctuate during the course of a connection between the base station and a mobile station, the slow power control loop can adjust the SIR target that the fast power control loop uses to adjust the base station""s transmitted power. Similar techniques can be used to control uplink transmit power.
FIG. 2 illustrates the physical relationship between the base stations of two cells A and B and their relative transmit power levels. In a conventional wireless communication system, base station 210A transmits signals to mobile stations which are located within cell A, as defined by the cell border 250. The cell border 250 can be a point where the strength of the signals transmitted at full power from base station 210A equals the strength of the signals transmitted at full power from base station 210B.
When a mobile station which is currently communicating with base station 210A moves over the cell border 250 into cell B, the mobile station will continue to communicate with base station 210A until the mobile station crosses hysteresis boundary 220B. The area in cell B from cell border 250 to hysteresis boundary 220B is known in the art as the hysteresis zone. The hysteresis zone is used by the wireless communication system to avoid a xe2x80x9cping-pongxe2x80x9d handoff effect, i.e., a mobile station which continuously hands-off based solely upon which base station is providing a greater signal strength at a particular instant of time. Accordingly, the mobile station will continue to communicate with base station 210A until there is a more significant change in the relative strength of the signals transmitted from base stations 210A and 210B. Typically the hysteresis zone is set such that there is approximately a 3-5 dB difference in the signal strengths transmitted from base station 210A and base station 210B. Although FIG. 2 illustrates the hysteresis zone as a uniform area surrounding the cell border 250, one skilled in the art will recognize that hysteresis is typically implemented in wireless communication systems by adding a predetermined signal strength value to the signal strength of the current connection. Hence, the actual area of the hysteresis zone will depend upon signal propagation conditions.
Now that the concept of hysteresis has been explained, a brief overview of a conventional handoff from base station 210A to base station 210B is presented below. When a mobile station in communication with base station 210A is moving towards base station 210B, the mobile station maintains communication with base station 210A until the mobile station crosses over the end of the hysteresis zone at hysteresis boundary 220B. When the mobile station crosses hysteresis boundary 220B, the mobile station is handed off to base station 210B. In typical wireless communication systems, since the base station does not know how far the mobile station is from the base station, the new base station begins transmitting signals to the mobile station at full power. For example, when a mobile station is handed off from base station 210A to base station 210B, base station 210B begins transmitting signals to the mobile station at full power such that the signals transmitted from base station 210B provide an acceptable quality signal out to the end of the hysteresis zone at hysteresis boundary 220A. However, as described above, mobile stations are typically located inside the hysteresis zone at handoff, not at the far boundary of the hysteresis zone. Accordingly, transmitting signals to a mobile station which has just undergone handoff at full power unnecessarily increases the interference in cell A and other nearby cells (not shown).
A second situation where a base station unnecessarily transmits to a mobile station at full power is during call set-up. Call set-up occurs when a mobile station either receives an incoming call or places an outgoing call. Since, in these situations, a mobile station can be located anywhere in the cell, the base station initially transmits signals to the mobile station at full power to increase the likelihood that the call set-up with the mobile station is successful.
FIG. 3 illustrates a cell 350 containing a base station 310 and a mobile station 390. When mobile station 390 initiates a call, the base station 310 initially begins transmitting to mobile station 390 at full power. The base station transmits at full power to ensure an acceptable signal quality in the region between the cell border 350 and the edge of the hysteresis zone 320. By transmitting at full power, signals from base station 310 actually propagate beyond the hysteresis boundary 320 to, for example, boundary 330. Boundary 330 is exemplary and merely illustrates where the signal power from base station 310 falls below the noise floor. One skilled in the art will recognize that the signals from base station 310 will in practice propagate infinitely. However, as described above, transmitting at full power generally increases interference without providing a substantial increase in the success of the call set-up between the base station and mobile station.
Accordingly, it would be desirable to provide a radiocommunication system in which base station power levels are minimized at call set-up and handoff to reduce system interference and to increase capacity. It would also be desirable to provide a radiocommunication system where system interference is reduced without increasing the amount of system resources used in the reduction of system interference.
These and other problems, drawbacks and limitations of conventional techniques are overcome according to the present invention, wherein less than full power can be used to transmit signals by base stations at call set-up and/or handoff. According to a first exemplary embodiment, the initial, individual transceiver power output for signals to be transmitted to a mobile station being handed off to that base station can be set to, for example, 2-5 dB less than the base station""s full transmit power, wherein the reduction in transmit power is a function of the depth of the hysteresis zone associated with both the cell handing off the call and the cell receiving the handoff. Further, the specific power reduction relative to full transmit power can be a fixed number within the aforementioned 2-5 dB exemplary range or may be variable (and outside of this range) based on factors such as a mobile station""s environment, system loading, quality reports from the mobile station. Moreover, in sectorized cells which have different size hysteresis zones for each sector, the reduction in power level will vary depending upon which sector the mobile station is located.
According to another exemplary embodiment of the present invention, the initial base station transceiver output power at call set-up may be reduced to less than the base station""s full output power. The specific power reduction relative to the full transmit power could be a function of a value associated with the hysteresis zone in the particular cell where the call is being set-up. In addition, the specific power reduction relative to the full transmit power can be a fixed number within a 2-5 dB exemplary range or may be variable (and outside of this range) based on factors such as a mobile station""s environment, system loading and quality reports from the mobile station.
An object of the present invention is to provide a method and apparatus for reducing interference in a power controlled, radiocommunication system.
Another object of the present invention is to provide a method and apparatus for reducing the amount of power transmitted from a base station to a mobile station at call set-up and/or call handoff.
Yet another object of the present invention is to reduce the amount of power transmitted from a base station to a mobile station at call set-up and/or call handoff without increasing the amount of system resources required for the reduction in power.
A further object of the present invention is to provide techniques for lowering the requirements for power matching between base stations, since the full power levels won""t be used during the initiation of at least most handoffs or call set-ups.
The above, and other objects of the present invention are achieved according to exemplary embodiments of the present invention by a method for initiating communication between a base station and a mobile station in a radio communication system. A base station is notified whether a mobile station requires support. The base station then determines whether the mobile station requires support for a call handoff or for a call set-up. A reduced initial power level is determined for transmission from the base station to the mobile station, where the value of the reduced power level is a function of whether the mobile station requires support for a call handoff or for a call set-up. The base station then transmits to the mobile station at the reduced power level. Since the reduced power levels do not require measurements to be made by the base station and/or the mobile station the amount of system resources used is not increased by the inventive system.